This invention relates to improvements in the performance of fluidized beds and techniques for fluidizing solids which heretofore could not be fluidized in a fluidized bed.
Fluidization of solids in a fluidized bed is a much used method of gas-solids contacting with many commercially successful applications in widespread fields. Fluidized beds find use in chemical reactors such as fluidized-bed catalytic crackers. Other chemical processes utilizing fluidized beds include processes for chlorination of hydrocarbons, oxidation of gaseous fuels, roasting of ore to facilitate release of valuable metals, calcination of lime in dolomite, and calcination of phosphate rock.
Fluidized beds are also used for physical contacting processes, such as for heat transfer, solids mixing, drying, size enlargement, size reduction, classification, adsorption, desorption, heat treatment, and coating. Exemplary of these processes are drying coal, cement, rock, and limestone, as well as coating metal parts with thermoplastic resins where a heated metal part is dipped into a fluidized bed of the thermoplastic resin.
An advantageous feature of fluidized beds is that fluidized beds tend to have nearly uniform temperatures and good heat transfer, both from gas to solids and from solids to internal surfaces. Further advantages are that addition or loss of solids through chemical or other means can normally be tolerated, and that rather thorough solids mixing occurs.
There are also, however, features of fluidized beds which are generally disadvantageous, and most of the disadvantages are due to bubbles. Bubbles provide a mechanism for gas bypassing, which can result in process inefficiency. The motion of the bubbles also promotes elutriation of fine particles, fragmentation of friable solids, and erosion of reactor surfaces.
Another problem experienced with fluidized beds is entrainment. As the fluidization velocity in a fluidized bed increases, entrainment of solid particles from the bed also increases, resulting in loss of material from the vessel containing the bed. This in turn increases operational costs to supply makeup material and/or to provide equipment to return entrained material to the bed.
Advantages and disadvantages of fludized beds are most easily presented in terms of the powder classification technique described in Geldart, "Types of Gas Fluidization", Powder Technology, 7 (1973) 285-92. Geldart divides solid particles into groups, based on particle density and particle size, as shown in FIG. 1.
Group A materials have a small mean particle size, typically 30 to 100 microns. Most commercial fluidized catalytic operations, such as catalytic cracking, are performed with materials of this size. Beds of group A material tend to bubble freely. Moderate bed expansions in the range of about 20 to about 50% can be attained. Bubbles tend to split and recoalesce frequently. Beds of group A materials noticeably expand before bubbling commences.
Group B materials include materials having a mean particle size ranging from about 100 to about 500 microns and a particle density of from about 1.4 to about 4 g/cm.sup.3. Sand is a typical group B powder. Fluidized coal combustion is an example of the use of group B material. When a group B material is fluidized, bubbles tend to be larger and more distinct than in group A materials, and there is no known limit to bubble size. Bed expansion commences with the onset of bubbling.
Group C materials are those which are in any way cohesive and generally can be fluidized only poorly or not at all. They are also known as cohesive powders. Gas tends to pass through the bed in the form of channels, resulting in poor gas/solid contact and little solids motion. Thus, in the current state of the art, Group C materials generally cannot be used in fluidized beds.
Group D material are large and/or very dense. A typical application for these very coarse materials is grain drying. Fluidization of group D materials is often performed in a spouted bed which uses a special gas distribution technique.
In view of problems associated with fluidized beds, there is a need for a technique that will improve the quality of fluidization of group A and group B materials, which are the materials most commonly used in fluidized bed processes, and for a technique which will permit good quality fluidization of group C cohesive powders.